>>41653877A key aspect of JavaScript development are closures: anonymous functions that capture variables from parent scopes.
due to implementation details of the JavaScript runtime, it is possible to leak memory in a subtle way
This snippet does one thing: every time replaceThing is called, theThing gets a new object which contains a big array and a new closure (someMethod). At the same time, the variable unused holds a closure that has a reference to originalThing (theThing from the previous call to replaceThing). Already somewhat confusing, huh? The important thing is that once a scope is created for closures that are in the same parent scope, that scope is shared. In this case, the scope created for the closure someMethod is shared by unused. unused has a reference to originalThing. Even though unused is never used, someMethod can be used through theThing. And as someMethod shares the closure scope with unused, even though unused is never used, its reference to originalThing forces it to stay active (prevents its collection). When this snippet is run repeatedly a steady increase in memory usage can be observed. This does not get smaller when the GC runs. In essence, a linked list of closures is created (with its root in the form of the theThing variable), and each of these closures' scopes carries an indirect reference to the big array, resulting in a sizable leak.
